In a photolithography process, the layout patterns on a photolithographic mask are transferred to the photoresist layer formed on a semiconductor substrate by an optical exposure process. The layout patterns may be chip patterns or align marks, etc.
FIG. 1 illustrates an existing photolithographic mask. As shown in FIG. 1, the layout patterns on the photolithographic mask includes dense features 1 and isolated features 2. The dense features 1 include a plurality of parallel first stripes 11; and the isolated main features 2 include a plurality of second stripes 21. The distribution density of the first stripes 11 are greater than the distribution density of the second stripes 21. That is, the distance between adjacent first stripes 11 is greater than the distance between adjacent second stripes 21.
During a photolithography process, the exposure light transmits through the transparent areas of the photolithographic mask to expose the photoresist layer formed on a semiconductor substrate. The exposure light is scattered in the photoresist layer; and scattering light is formed. The edge portions of the photoresist layer corresponding to the first stripes 11 and the second stripes 21 are affected by the scattering light. However, due to the difference between the distribution density of the first stripes 11 and the second stripes 21, the exposure window (transparent area) size between adjacent first stripes 11 are smaller than the exposure window size between adjacent second stripes 21. Thus, the effect to the edges of the first stripes 11 caused by the scattering light is different from the effect to the edges of the second stripes 21 caused by the scattering light. Therefore, the error of the feature size of the first stripes 11 are different from the error of the feature size of the second stripes 21.
In order to solve the error difference between the first stripes 11 and the second stripes 21, referring to FIG. 1, at least one Sub-Resolution Assistant Feature (SRAF) 22 is formed between adjacent second stripes 21. The SRAFs 22 are also stripes to cause that the distribution density of the second stripes 21 and the SRAFs 22 are identical to the distribution density of the first stripes 11. Thus, the exposure window size in the dense patterns 1 and the exposure window size in the isolated patterns 2 are similar. During an exposure process, the SRAFs 22 are able to compensate the effect to adjacent second stripes 22 caused by the scattering light. Therefore, the feature size error of the first stripes 11 may be identical to the feature size error of the second stripes 21. Further, the feature size of the SRAFs 22 is smaller than the resolution of the exposure apparatus, the SRAFs 22 are not transferred into the photoresist layer, thus the SRAFs may be referred as non-printable SRAFs.
The method for forming the photolithographic mask having the layout patterns illustrated in FIG. 1 includes sequentially providing a substrate having a mask layer; forming a photoresist layer on the mask layer; patterning the photoresist layer to define the position of the layout patterns; forming the layout patterns by a dry etching process to etch the mask layer using the patterned photoresist layer as an etching mask; cleaning the residues of the dry etching process, such as reaction products, or contaminations, etc., using a cleaning agent; and spin-drying the substrate to remove the residual cleaning agent. Because the feature size of the SRAFs 22 is substantially small, during the spin-drying process, the SRAFs 22 may be tilt, and/or peel off. Thus, the SRAFs 22 may be unable to compensate the scattering light during the exposure process. The disclosed structures and methods are directed to solve one or more problems set forth above and other problems.